Method of manufacturing elemental phosphorus

ABSTRACT

Electric furnace processes for the manufacture of elemental phosphorus are substantially improved by the use of amorphous carbon electrodes having sonic propagation velocities of from 5,000 to 5,900 feet/second.

United States atent Milton P. Albert;

Milton J. Scott, both of St. Louis, Mo. 878,208

Nov. 19, 1969 Jan. 4, 1972 Monsanto Company St. Louis, Mo.

inventors Appl. No. Filed Patented Assignee METHOD OF MANUFACTURING ELEMENTAL PHOSPHORUS 1 Claim, No Drawings U.S. Cl 23/223,

' 204/294 Int. Cl C0lb 25/00 Field of Search 23/223;

[5 6] References Cited UNITED STATES PATENTS 3,010,796 ll/l96l Alexander et al 23/223 Primary Examiner-Oscar R. Vertiz Assistant ExaminerCharles B. Rodman Attorneys-Herbert B. Roberts, Roger R. Jones, Thomas N.

Wallin and Neal E. Willis ABSTRACT: Electric furnace processes for the manufacture of elemental phosphorus are substantially improved by the use of amorphous carbon electrodes having sonic propagation velocities of from 5,000 to 5,900 feet/second.

METHOD OF MANUFACTURING ELEMENTAL PHOSPI-IORUS BACKGROUND OF THE INVENTION This invention relates to an improved electric furnace process for the manufacture of elemental phosphorus. In particular, the invention relates to such a process characterized by the exclusive use of furnace electrodes which are uniquely resistant to breakage.

As is well known to those skilled in the art, essentially all elemental phosphorus is now industrially produced by electric furnace reduction of phosphatic raw materials in the presence of silica and carbon (principally supplied in the form of coke or calcined anthracite coal). Processes of this type are described, for example in Volume II, Phosphorus and its Compounds, edited by John R. Van Wazer, lnterscience Publishers Inc., 1961. In these processes, submerged electrical arcs between the tips of large electrodes (40 inches or more in diameter) and the furnace floor provide thermal energy for reducing the phosphatic feed stock to elemental phosphorus and vaporizing the phosphorus from the furnace for condensation and collection.

Breakage of electrodes utilized in such processes constitute a major practical problem. Such breakage occurs primarily in the lower region of the furnace where high thermal stresses are encountered and such breakage results in the electrode tip being positioned at a higher level within the furnace. (The broken electrode pieces or sections falling to the furnace floor effectively raises the floor height at that point. Thus, the overall effect is to raise the height of the electric arc in the furnace.) The higher tip position results in increased temperature of off gases which now pass upwardly a reduced distance through the bed of the cooler feedstock which is continuously moving downward. In order to prevent damage to the furnace, and other deleterious effects, it is necessary to reduce the temperatures of off gases resulting from the higher electrode tip position by reducing the electrical power input or by other costly means which lower the yield of phosphorus and thereby increase cost of production.

It is thus apparent that an electric furnace process characterized by reduced electrode breakage would constitute a significant advancement in the art of producing elemental phosphorus.

SUMMARY OF THE INVENTION It is an object of this invention to provide an electric furnace process for the manufacture of elemental phosphorus improved in and characterized by a substantial reduction in electrode breakage. This object is accomplished by the use of amorphous carbon electrodes having diameters of at least 40 inches and characterized by sonic propagation velocities of from 5,000 to 5,900 feet/second in otherwise conventional electric furnace processes.

The invention will be better understood from the following description of the preferred embodiments.

DESCRIPTION OF THE PREFERRED EMBODIMENTS This invention is based on the unexpected discovery that the utilization in electric furnace processes of electrodes having certain critical characteristics provides significant advantages in the production of elemental phosphorus.

Carbon electrodes of the type used in phosphorus furnaces are prepared by forming mixtures of anthracite coal, graphite, and coal tar pitch; molding or extruding such mixtures into substantially cylindrical shapes of desired dimension and curing the formed mixture by heat treating to a temperature of from about 850 to 950 C. The heating and cooling cycle generally requires 30 to 60 days. It has been discovered that commercially available electrodes produced in this manner have sonic propagation velocities (as will be hereinafter defined) of from about 5,000 to 7,000 feet/second. The discovery that those electrodes characterized by sonic propagation velocities of from 5,000 to 5,900 feet/second are uniquely resistant to breakage when used in electric furnace processes is particularly unexpected in unexpected in view of the fact that such electrodes possess substantially lower intrinsic physical strength than those having higher sonic propagation velocities since it was previously considered that resistance to breakage was directly proportional to physical strength.

The phrase sonic propagation velocity" is used in the specification and claims to denote the velocity at which sound waves of 24 kilohertz frequency are propagated between points on the electrode surface circumferentially spaced by the arc distance subtending a l20 angle. The distance traveled by the sound is, of course, the distance of the chord of that arc. Measurement of sonic propagation velocity can be accomplished by locating circumferentially spaced points on the electrode corresponding to the ends of a 120 arc. An electroacoustic transducer for converting an electrical impulse to a 24 kilohertz acoustic signal is placed at one of these points and a transducer for converting a received acoustic signal to an electrical signal is placed at the other point. Good acoustical coupling is ensured by the presence of a viscous liquid, such as an aqueous carboxymethyl cellulose solution between the electrode surface and the transducer faces. An electrical impulse is applied to the sound-generating transducer and the time required for transversal of the first signal (the longitudinal sound wave) to the receiving transducer is measured by conventional oscilloscopic techniques. Sonic propagation velocity is routinely calculated from the data obtained. The measurements are preferably repeated at several circumferential positions for each of several longitudinal positions on the electrode to statistically ensure that maximum and minimum sonic propagation velocities are determined.

It is to be further understood that designation of a specified sonic propagation velocity range is intended as a specification of maximum and minimum values as determined by measurements such as described above.

Also it is to be understood that the sonic propagation velocity characteristics set forth refer to propagation rates measured at ambient temperature prior to use of the electrode in the furnace.

Generally, the electric furnaces commercially employed for production of elemental phosphorus utilize a plurality of electrodes each of which comprises a plurality of longitudinally contiguous sections. These sections are connected by means of threaded nipples which fit sockets in the adjacent sections, or by other conventional means. In the practice of this invention, it is essential that every section of each electrode possess the critical characteristics described above. The advantages of the invention are not obtained when only random electrode sections possess these critical characteristics.

The invention is further illustrated by the following examples.

In these examples, elemental phosphorus is produced in an electric furnace fitted with three amorphous carbon electrodes each comprising a plurality of electrode sections 55 inches in diameter connected by nipples screwed into sockets in the ends of each electrode section, and having sonic propagation velocity characteristics as described in each example. The raw material burden fed to the furnace is a conventional feedstock comprising nodulized phosphate ore, coke, and silica. The chemical composition and physical characteristics of the burden are maintained substantially constant during furnace operation. In each example, the furnace is operated continuouslyexcept for shutdowns required for routine maintenance, adjustment of electrode tip position, and addition of electrode sections-for a period of days. Electrical power of 50,000 kilovolt-amperes is fed to the fur nace at all times except when off gas temperature rises to about 500 C. necessitate lowering the power to reduce temperature in order to prevent damage to the furnace. Such rises in off gas temperature result when electrode breakage occurs which raises the position of the electrode tip, thereby reducing the distance of off gas travel through the cooler descending feedstock. This condition cannot be corrected by adjusting electrode position since the broken portion of electrode falls to the furnace floor effectively raising its height and repositioning the electrode tip would result in an improper arc distance. Accordingly, it is necessary to operate under reduced power until the broken section of electrode is consumed or displaced from beneath the electrode tip by shifting of the furnace burden.

EXAMPLE I The furnace is fitted with electrodes having minimum sonic propagation velocities of 5,500 feet/second and maximum sonic propagation velocities of 5,900 feet/second.

These velocities are determined by measurements made every 12 inches of electrode length. At each such longitudinal position, the propagation rate of sound along the chords of twelve 120 arcs circumferentially spaced by 30 is measured.

During the 90 days of phosphorus production, the off gas temperature does not rise above 500 C. and current reduction is not required. This indicates the absence of major electrode breakage during operations.

Operation is continuous except for 2.7 days required for routine furnace maintenance and 0.45 days required for electrode position adjustment and section addition; (necessitated by normal electrode bum-off rather than breakage). 17.4 million pounds of elemental phosphorus is produced.

EXAMPLE I] FOr purposes of comparison, the procedure of example I is repeated with the exception that the furnace is fitted with electrodes having minimum sonic propagation velocities of 5,500 feet/second and maximum sonic propagation velocities of 6,500 feet/second. Ninety-three percent of sound propagation measurements made as in example I result in measured sonic propagation velocities of above 5,900 feet/second. During 90 days of phosphorus production, it is necessary to reduce the current 48 times to compensate for off gas temperature rises above 500 C. caused by electrode breakage. During the -day period, it is necessary to maintain current at reduced levels (just sufficiently low to maintain temperatures below 500 C.) for a total of 3 days. A total downtime of 1 day is required for electrode position adjustment and section addition. (The higher time required for this operation as compared to example I is due to the more frequent adjustments and section additions required to compensate for breakage.) Downtime of 6.9 days is required for routine maintainance. This higher downtime, as compared to example i, is in part due to higher off gas temperatures resulting from electrode breakage.

Phosphorus production over the 90-day period is only l6.2 million pounds.

EXAMPLE ill The procedure of example I is repeated with the exception that electrodes having minimum sonic propagation velocities of 5,000 feet/second and maximum sonic propagation velocities of 5,800 feet/second are utilized. Similar results are obtained.

it is seen from the foregoing examples, that the use of electrodes having sonic propagation velocities between 5,000 and 5,900 feet/second provide substantial advantages in elemental phosphorus production.

What is claimed is:

1. In a process for manufacturing elemental phosphorus by reducing phosphatic raw materials in the presence of silica and carbon in an electric furnace having a plurality of Ratterally spaced amorphous carbon electrodes each of said electrodes comprising a plurality of longitudinally contiguous sections, the improvement wherein the sonic propagation velocity of each section of each electrode is from 5,000 to 5,900 feet/second. 

